Protective automatically disconnectable grouping means for an ungrounded a.c. electrical distribution system

ABSTRACT

An ungrounded a.c. electrical distribution system is provided with a protective grounding device in circuit with means for automatically disconnecting the grounding device in the event that a fault to ground occurs on any line of the system. The grounding device can be a transformer having a neutral grounded point. The automatic disconnection means may be circuit breakers or current limiting fuses and may further include a relay assembly having automatically opening contacts. The relay assembly can be thermally operable. The transformer can be a zigzag transformer. External signal devices can be connected to the relay assembly.

United States Patent [451 July 18,1972

Beachley [72] lnventor: Robert W. Beachley, PO. Box 11281,

Charlotte, NC 28209 [22] Filed: Feb. 25, 1971 21 App]. No.: 118,700

1,731,971 10/1929 Courtinetal. ..317/l8R Primary Examiner-James D.Trammell Attorney-Polachek, Saulsbury & l-lough [57] ABSIRACT Anungrounded a.c. electrical distribution system is provided with aprotective grounding device in circuit with means for automaticallydisconnecting the grounding device in the event that a fault to groundoccurs on any line of the system. The grounding device can be atransformer having a neutral grounded point. The automatic disconnectionmeans may be circuit breakers or current limiting fuses and may furtherinclude a relay assembly having automatically opening contacts. Therelay assembly can be thermally operable. The transformer can be azig-zag transformer. External signal devices can be connected to therelay assembly.

10 Claim, 3 Drawing Figures UNGROUNDED 3-PHASE LOAD [52] U.S. Cl..317/l8 R, 3l7/40A [51] Int. Cl. ..H02h 3/16 [58] FleldofSearch..3l7/l8 R, 18 D,40A

[56] References Cited UNITED STATES PATENTS 2,523,778 9/1950 Randall..3l7/18 R FROM 12 I4 UNGROUNDED P 3-PHASE VOLTAGE SOURCE SIGNAL ORCONTROLLED DEVICE I l l C so I {03 PAIENIEI] MI 8 I972 FIGOZ. 3.

FROM UNGROUNDED' j UNGROUNDED B-PHASE UNGROUNDED I7 3 PHASE 9 3LFg-LD6SELOAD VOLTAGE T' I I I I68 D D 3 l2 4 l I UNGROUNDED I I 3-PHASE sOuRcE 522 I8 THERMAL OvERLoAO RELAY ASSEMBLY 24 ZlG-ZAG TRANSFORMER ZIG-ZAG P LJ TRANSFORMER i 30 I O6 T IOA gl? .FROM UNGROUNDED UNGROUNDED s-PHAsE3,-PHAsE VOLTAGE LOAD sOuRcE r-""'1 C 1 Cl I SIGNAL OR CONTROLLED DEVICEI I I I {c2 C3 I l L -1 INVENTOR.

ROBERT W. BEACHLEY WW ATTURNE Y5 PROTECTIVE AUTOMATICALLY DISCONNECIABLEGROUPING MEANS FOR AN UNGROUNDED A.C. ELECTRICAL DISTRIBUTION SYSTEMThis invention concerns means providing an ungrounded a.c. electricaldistribution system with a neutral grounding device and means forautomatically disconnecting the grounding device upon occurrence ofground fault of sufficient magnitude in the system.

The present invention is applicable to alternating current electricaldistribution system in which all conductors are insulated from earth.The invention provides a reference ground to the system so that innormal operation with no electrical ground fault in the system, theoperating characteristics of the system are the same as in a groundedsystem. The reference ground is connected to the system throughautomatically operated disconnection means which disconnect thegrounding means from the system if a ground fault of sufficientmagnitude occurs to operate the automatic disconnection means. By thisarrangement the system will continue to deliver power even though oneconductor or phase of the system is grounded by a ground fault.

Currently three-phase alternating current systems such as employed inindustrial power plants, are generally electrically balanced. Each phaseis electrically displaced from the other two phases by 120 electricaldegrees. Such electrical systems have been operated grounded orungrounded. The grounded system generally employs a solid directconnection between a neutral point or line of the system and earth. Thismay be a connection to water piping, a ground grid, or other such directgrounding means. An ungrounded system has all of its electricalconductors insulated from earth and operated at some potential fromearth. The grounded system has the advantage that in the event oftransient overvoltages, damages to loads on the line will be minimized.This provides greater safety to personnel and reduces operating andmaintenance expense incurred by users of the system. The ungroundedsystem has the important advantage that it can continue to deliver powerto power consuming loads and equipment in the system even though oneconductor is accidentally grounded.

Frequently, during some system change, an existing ungrounded electricaldistribution system is centrally grounded, changing the utility deliveryvoltage from the characteristics of delta ungrounded to that of wye,center grounded. This transformation may be accomplished by theconnection of any type of electrical load connected in wye with thecentral point grounded, to the delta ungrounded service. Such a load canbe resistive, reactive, capacitive or a combination of these, with thephase values balanced to properly locate the ground connection atneutral. The generally preferred method is to use inductive grounding inthe form of a wye-delta bank of transformers, a T-connected transformer,or a n'g-zag transformer. The use of all of these types of connectionfor establishing a grounded neutral on a previously ungrounded a.c.system is well known and frequently used. The installation of suchgrounding means, permanently converts the system from an ungroundedsystem to a grounded neutral system. In such conversions, the groundingequipment whether transformers or other means is permanently connectedand of such size that it can safely carry any expected ground faultcurrent. Although the grounding equipment may be connected to the systemthrough its own disconnect means, the grounding equipment always remainsconnected to the system during the occurrence of all ground faultsregardless of their magnitudes. As a result of grounding the system, theadvantage of continued system service is lost in the event of occurrenceof a ground fault.

The present invention makes it possible to obtain the benefits of bothan ungrounded and a grounded system. During normal operation the systemoperates as a grounded neutral system. If a ground fault occurs in thesystem, its grounding is removed automatically and the system continuesto deliver power while measures are taken to locate and clear plished byconnecting a zig-zag transformer with neutral grounded to the powerdistribution system. The zig-zag transformer is located as close aspossible to the source of power and in advance of ground fault detectionequipment. While a zig-zag transformer is preferred, other types ofgrounding devices can be used. The grounding device whatever itsconstruction, is connected to the power-distribution lines through meansfor automatically cutting out the grounding device in the event that aground fault occurs. This automatic cutout device is preferably a relaywith themial overload contacts which open in the event a ground faultcurrent occurs having a magnitude above the safe current carryingcapacity of the grounding means. When the relay opens due to occurrenceof a ground fault of sufficient magnitude, the zig-zag transformer orother grounding device is disconnected from the system, and the systemcharacteristics revert to those of an ungrounded system with only onephase grounded by the ground fault. This permits continued operation ofthe power distribution system until the ground fault can be located andcorrected by repairmen.

The zig-zag transformer is preferably chosen of such size and currentcarrying capacity, that it will stay connected through minor, nuisanceground faults and will only be cut out when a serious ground faultoccurs in any phase of the system.

Two important benefits will result from using a neutral grounded zig-zagtransformer in an otherwise ungrounded electrical distribution system.First, an intermittent ground, such as a conductor lightly brushing aconducting surface, for example, will exhibit arcing due to the groundpath provided by the zig-zag transformer, and will thus be more easilylocated by visual examination of the system. Second, the entire systemcan continue to operate until proper measures are taken to correct aground fault located by proper ground fault detection means.

The invention is applicable to single-phase and two-phase as well as tothree-phase systems. Other and further features, objects and advantagesof the invention become apparent from the following detailed descriptionof the invention taken together with the accompanying drawings, wherein:

FIGS. 1, 2 and 3 are diagrams of three electrical power dis-- tributionsystems including neutral grounding means with different arrangements ofautomatic cutout means for the grounding means.

Referring first to FIG. 1, there is shown schematically a three-phaseelectrical distribution system 10 providing a.c. electrical power inthree phases on three wire conductors l2, l4 and 16. The conductorsterminate at a suitable load 17 capable of utilizing three-phaseelectric power. A switch 18 is provided having fixed contacts 20connected to the respective conductors 12, 14 and 16 and three gangedpoles 22 actuated by control arm 24 to close with contacts 20respectively. Connected to the respective switch poles are fuses 24. Arelay assembly 25 is further provided in the system. Thisrelay assemblyhas heat sensitive elements 26 respectively connected in series with theswitch poles 22, fuses 24 and normally closed relay contacts 28, 29.Zig-zag transformer 30 is connected to relay assembly 25. A normallyclosed switch 27 is connected in series with relay coil 31. This relaycoil holds contacts 28, 29 in closed position when it is energized uponclosure of switch 18. Coil 31 is connected to one pair of conductors 14,16 via switch 18. All the heat sensing elements are arranged to operateswitch 27 to open it if any one heat sensitive element 26 passes toolarge a current. Switch 27 will deenergize holding coil 31. When holdingcoil 31 becomes deenergized, contacts 28 and 29 open in all three phasesof the distribution system, thus completely disconnecting zig-zagtransformer 30 from lines 12, 14 and 16.

Transformer 30 has a magnetic core 32 with three legs 33, 34 and 35.Coils 36, 38 are wound on leg 33 in opposite directions; coils 40, 42are wound on leg 34 in opposite directions and coils 44, 46 are wound onleg 35 in opposite directions. Coil 36 is connected in series with coil42. Coil 40 the ground fault. The purposes of the invention are accomisconnected in series with coil 46. Coil 44 is connected in series withcoil 38. Coils 38, 42 and 46 are connected at point P to ground or earthG. Thermal elements 26 of relay 25 are connected respectively to coils36, 40 and 44. By the arrangement described a neutral ground iseffectively applied to the three-phase circuit 12, l4, 16, when switch18 is closed.

The system now operates normally as one connected to a neutral ground.The advantages of such operation are well known and have been pointedout briefly above. Suppose now that a minor temporary grounding fault Sto earth occurs on any line conductor, for example at point P on lineconductor 16. Before the ground fault occurred, the potential at point Pwas at the electrical center of the three-phase voltages and the valueof this potential from any phase to ground was the phase-to-phasevoltage divided by the square root of three. Now, when a ground faultoccurs at point P, current will flow into the ground, into the steel ofa building housing the equipment, or any convenient path back to pointP, through the zigzag transformer to the other two phases l2 and 14. Acurrent flowing from point P to ground and attempting to reach phase 12must pass through coil 36, but to get to coil 36 it must pass from pointP through coil 42. All six coils of the zigzag transformer have the samenumber of turns. Therefore, a current in coil 36 will induce (bytransformer action) a current of like magnitude in coil 38, and acurrent in coil 42 will induce a like current in coil 40, and so on,with the result that the ground fault current at P is divided into threeequal parts, each part flowing back to one of the phases. The result isthat the zig-zag transformer will attempt to keep the potential of allthree phases the same to ground. The amount of current that will flow toground from point P will be determined by the circuit characteristicsand the solidness of the ground at point P. If the zig-zag transformeris left connected to the circuit, the amount of current flowing toground at point P may become enough to interrupt the flow of power tothe load by blowing circuit fuses or tripping a circuit breaker; or byheating at point P, a fire may start before the circuit is interrupted.However, if the zig-zag transformer can be automatically disconnectedfrom the circuit when a preset value of ground fault current is reached,the ground fault return path will be broken and power flow to the loadneed not be interrupted by operation of normal circuit protectivedevices.

Relay assembly 25 and transformer 30 are protected by fuses 24. Theaccidental grounding of point P may be so severe and sudden that thethermal response of relay assembly 25 is not quick enough to preventdamage to the relay assembly and or the transformer. For this reason,fuses 24 are in the circuit and will blow before damage can result.Further, because of the fuse protection, the transformer and the relayassembly need not be of size sufiicient to safely experience groundfaults of any magnitude, but need be only of such size as to continueoperation through small nuisance ground faults that neet not cut off thezig-zag transformer. if the surge in current is very large and rapid andoccurs before relay 25 can respond, any one or all of fuses 24 willopen. In this way the grounding will be limited to the ground faultcondition at point P and remedial measures can be instituted accordingto some prearranged plan and schedule to locate and correct the groundfault. Since the system conductors l2, l4 and 16 are now ungrounded,except for the ground fault at point P, power will continue to flow tothe load 17. Thus the power supply to the load remains uninterruptedwhether the fault to ground at point P is small or large. In addition,the transformer 30 is prevented from burnout since the fuses 24 willblow before any damage occurs to any of the transformer windings.

System A shown schematically in FIG. 2 is similar to system 10 of FIG. 1and corresponding parts are identically numbered. Here circuit breakers50 replace the switch 18 and fuses 24 of system 10. The thermal overloadrelay assembly 25 and zig-zag transformer 30 operate in the same way asexplained above in connection with system 10. All of the circuitbreakers 50 will open in order to protect the windings of the zig-zagtransformer from burnout if a prolonged fault to ground occurs at anyone line 12, 14 and 16.

System 108 shown schematically in FIG. 3 dispenses with the thermaloverload relay assembly 25 of systems 10 and 10A but continues use ofswitch 18 and fuses 24. The fuses will blow to protect the windings ofthe transformer 30 and to disconnect the lines 12, 14 and 16 from groundvia the transfonner if a fault to ground occurs as explained above.System 108 will be useful on high voltage systems where the initial costof relay assembly 25 can be avoided. Fuses 24 will blow for largerground fault currents and will disconnect the grounding means from thesystem as in system 10.

In systems 10, 10A and 108, the disconnecting of the grounding means isautomatic, but its reconnection is part of the work of repairmen, afterthey seek and eliminate the ground fault. The relay assembly could be ofan automatic reclosing type which would restore the grounding means tothe circuit should it happen that the ground fault cleared itself.However, it is considered safer and better practice that the groundingmeans remain disconnected until, by human intervention, the ground faultis noted, the reason for the fault is found and corrected.

Relay assembly 25 is provided with further contacts C1, C2 and C3, C3,arranged to be operated by the heat sensing elements 26 when contacts 28and 29 open. Contacts C1, C2 open and contacts C3, C3 close whencontacts 28, 29 open. These contacts can be connected to an externalsignal or control device 60 for signaling that the grounding means hasbecome disconnected, requiring the attention of repair or servicepersonnel. The signal device can be a horn, light or control device ofany desired type.

By the system arrangement described, the electrical distribution systemis enabled to take advantage of the best characteristics of both agrounded system and an ungrounded system.

Although a limited number of embodiments of the invention have beendescribed and specifically illustrated this has been by way of exampleonly. Many changes and modifications are possible. For example, a wyedelta transformer can be substituted for a zig-zag transformer. Thegrounding means and automatic cut-out for the grounding means can beapplied to a single-phase or a two-phase power distribution system.Other types of automatically opening relay assemblies can be used thanthe specific thermal overload relay assembly illustrated. Still othermodifications and variations are possible without departing from thespirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. Protective means for a normally ungrounded electrical distributionsystem having a plurality of ungrounded conductors connected to a load,comprising automatically openable current conducting means responsive toany current above a predetemiined magnitude connected to said conductorsrespectively and arranged to open when current in any one of saidcurrent conductive means rises above said magnitude; and a groundingdevice connectable and when in use connected between said conductivemeans and ground to provide a neutral ground for the ungroundedconductors, whereby said grounding device is automatically cut off fromsaid conductors when a fault to ground of suflicient magnitude andduration occurs between any one of said conductors and ground, saidautomatically opening current conductive means comprising for each ofsaid ungrounded conductors switch means and fuse in series circuits withsaid grounding device and in parallel with said load to said conductors,when said conductors are connected, and the automatically openingcurrent conductive means comprising a relay assembly includingautomatically opening current overload switch contacts in series withsaid grounding device and in parallel with said load to said conductors,when said conductors are connected.

2. Protective means as defined in claim 1, including said ungroundedconductors in an operatively connected state.

3. Protective means as defined in claim 1, wherein the automaticallyopening current conductive means further comprises current limitingfuses connected in said series circuits with said switch contacts andsaid ground.

4. Protective means as defined in claim 1, wherein said relay assemblyhas thermally responsive elements arranged to open said contacts inresponse to excessive current passing therethrough, wherein theautomatically opening current conductive means further comprises currentlimiting fuses connected in series circuits with said switch contactsand said ground, and wherein said grounding device is a transformerhaving a neutral grounding point.

5. Protective means as defined in claim 4, wherein said relay assemblyfurther includes means for connecting an external signal device theretofor indicating when said fault to ground occurs, and wherein saidtransformer is a zigzag transformer.

6. Protective means as defined in claim 1, wherein said relay assemblyhas thermally responsive elements arranged to open said contacts inresponse to excessive current passing therethrough.

7. Protective means as defined in claim 1, wherein said grounding deviceis a transformer having a neutral grounding point.

8. Protective means as defined in claim 7, wherein said transformer is azig-zag transformer.

9. Protective means as defined in claim 1, wherein said relay assemblyfurther includes means for connecting an external signal device theretofor indicating when said fault to ground occurs.

10. Protective means as defined in claim 9, wherein said groundingdevice is a transformer having a neutral grounding point.

1. Protective means for a normally ungrounded electrical distributionsystem having a plurality of ungrounded conductors connected to a load,comprising automatically openable current conducting means responsive toany current above a predetermined magnitude connected to said conductorsrespectively and arranged to open when current in any one of saidcurrent conductive means rises above said magnitude; and a groundingdevice connectable and when in use connected between said conductivemeans and ground to provide a neutral ground for the ungroundedconductors, whereby said grounding device is automatically cut off fromsaid conductors when a fault to ground of sufficient magnitude andduration occurs between any one of said conductors and ground, saidautomatically opening current conductive means comprising for each ofsaid ungrounded conductors switch means and fuse in series circuits withsaid grounding device and in parallel with said load to said conductors,when said conductors are connected, and the automatically openingcurrent conductive means comprising a relay assembly includingautomatically opening current overload switch contacts in series withsaid grounding device and in parallel with said load to said conductors,when said conductors are connected.
 2. Protective means as defined inclaim 1, including said ungrounded conductors in an operativelyconnected state.
 3. Protective means as defined in claim 1, wherein theautomatically opening current conductive means further comprises currentlimiting fuses connected in said series circuits with said switchcontacts and said ground.
 4. Protective means as defined in claim 1,wherein said relay assembly has thermally responsive elements arrangedto open said contacts in response to excessive current passingtherethrough, wherein the automatically opening current conductive meansfurther comprises current limiting fuses connected in series circuitswith said switch contacts and said ground, and wherein said groundingdevice is a transformer having a neutral grounding point.
 5. Protectivemeans as defined in claim 4, wherein said relay assembly furtherincludes means for connecting an external signal device thereto forindicating when said fault to ground occurs, and wherein saidtransformer is a zigzag transformer.
 6. Protective means as defined inclaim 1, wherein said relay assembly has thermally responsive elementsarranged to open said contacts in response to excessive current passingtherethrough.
 7. Protective means as defined in claim 1, wherein saidgrounding device is a transformer having a neutral grounding point. 8.Protective means as defined in claim 7, wherein said transformer is azig-zag transformer.
 9. Protective means as defined in claim 1, whereinsaid relay assembly further includes means for connecting an externalsignal device thereto for indicating when said fault to ground occurs.10. Protective means as defined in claim 9, wherein said groundingdevice is a transformer having a neutral grounding point.